Silicon−Carbon Unsaturated Compounds. 57. Photolysis of meso- and racemic-1,2-Diethyl-1,2-dimethyldiphenyldisilane, Direct Evidence for a Concerted 1,3-Silyl Shift to ortho-Carbon in the Phenyl Ring

meso- and racemic- 1,2-diethyl-1,2-dimethyldiphenyldisilane ( 2a ) and ( 2b ) were synthesized respectively by hydrogenation of meso- and racemic- 1,2-diethynyl-1,2-dimethyldiphenyldisilane in the presence of a IrCl(CO)(PPh 3 ) 2 catalyst. Irradiation of 2a with a low-pressure mercury lamp in the presence of isobutene in hexane proceeded with high diastereospecificity to give ( R,S )- and ( S,R )-2-(isobutylethylmethylsilyl)-1-(ethylmethylphenylsilyl)benzene in 77% yield. Similar irradiation of 2b with isobutene produced ( R,R )- and ( S,S )-isomer. The photolysis of 2a and 2b in the presence of 1,1-diphenylethylene also proceeded diastereospecifically to give the respective adducts. With 2,3-dimethylbutadiene, 2a and 2b produced the corresponding adducts, whose spectrometric analysis showed the presence of two diastereomers. Results on theoretical studies which were carried out using PhSiH 2 SiH 3 as a model have also been reported.